This project seeks to quantify the extent and time course of the bone loss produced by episodes of temporary illness of a sort associated with bed rest and catabolic influences. The underlying hypothesis is that recovery of bone lost under these circumstances is often incomplete, particularly in critical regions such as the spine and hip. If true, this would mean that some portion of age-related bone loss is a punctuated phenomenon, related to illness and/or immobilization, rather than being continuous and inexorable. This finding would open up additional opportunities for osteoporosis prevention. As a surrogate for a wide variety of catabolic episodes, the investigators will use routine, elective abdominal or pelvic surgery performed on individuals 55-70 years of age. They will measure bone mass by DEXA at spine and hip in 150 individuals before surgery, at 4-8 weeks post surgery, and then at 4, 8, and 12 months, to quantify and characterize the extent of bone lost under these circumstances. They will also measure various potentially modifying covariates, such as degree of temporary disability, Ca intake, hormonal therapy, vitamin D status, age, and life-style factors, any of which might plausibly influence extent of bone loss or degree of recovery. They will compare decreases at various time points after surgery to baseline values in each woman and will compare mean bone mass changes at 12 months with a non-operated control group matched for age and estrogen replacement therapy and measured twice across a 12-month interval.